Swirl injector for 4" hybird

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strud
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Swirl injector for 4" hybird

Postby strud » Fri Nov 15, 2019 12:10 pm

So after a significant amount of review of the literature around hybrid motor stability, I've decided to make some changes to the design of the 4" motor.

One thing that was found to significantly help reduce instability that was not fuel related is the addition of swirl to the oxidiser flow ie by having a swirl injector.

Making this is not straight forward but it has been an interesting challenge.

Due to the design of the injector housing, I have a 15mm disc that can be utilised to either be the injectors or as I found out after trying to drill small holes in at a compound angle, use it to house removable inserts.

The current tune for the motor calls for 8 x 1.6mm injectors hence I went with a 45deg spacing and the maximum inclination I could arrange on my mill with the rotary table installed that being 25deg. After a little bit of modelling I came up with a 14deg angle for bringing the streams towards the centre such that they would not impinge on the preheater insulation sleeve.

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Due to the compound angle, a number of machining steps are needed to drill the holes 'straight' and the give the injector heads (M5x16mm brass hex head bolts) room to pull down onto a flat.

In the end, it came out quite well

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But installing some longer screws it is easier to visualise the stream paths

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Will make up the injectors as soon as the brass bolts get here..... they are not easy to find these days!

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SpaceManMat
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Re: Swirl injector for 4" hybird

Postby SpaceManMat » Fri Nov 15, 2019 2:29 pm

Nice, can you show us a flow test with water when it’s done? Would be nice to see how it in action.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

strud
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Re: Swirl injector for 4" hybird

Postby strud » Fri Nov 15, 2019 4:05 pm

Yes want to do this anyway.

Water flow is obviously not the same but will give some idea......

strud
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Re: Swirl injector for 4" hybird

Postby strud » Tue Nov 19, 2019 6:36 pm

Have the brass bolts now, hope to get some time to drill & chamfer these shortly

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strud
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Re: Swirl injector for 4" hybird

Postby strud » Wed Nov 20, 2019 1:08 pm

Drilled at 1.5mm and entry chamfered

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Need to make sure all of the exits are deburred as well....

Next water visualisation test

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Re: Swirl injector for 4" hybird

Postby air.command » Wed Nov 20, 2019 7:06 pm

Looking great! Will there be a seal of some sorts under the bolt head, or is the leakage around the thread insignificant?
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strud
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Re: Swirl injector for 4" hybird

Postby strud » Wed Nov 20, 2019 8:22 pm

I hadn't planned to put any seal around those threads or under the bolt heads, but now that you've mentioned it and I've thought about it a bit more I may put a soft copper washer or similar under the bolt heads. The leakage through the thread would likely be insignificant but if there was any flow through there it could cause unwanted corrosion/oxidation etc making for a seized in bolt etc.

There is an oring under the bottom edge of the carrier disc.


I've been reading more about the effect of swirl on hybrid combustion and optimisation of injectors for hybrids. Lots of new research in this area over the last few years.

One of the main issues with axial injection is a fair bit of the N2O/oxidiser ends up making it to the end of the motor without much interaction with the fuel surface. By introducing swirl and NOT injecting axially in the centre of the port, combustion efficiency can be improved quite dramatically with the other benefit of helping reduce some of the instabilities.

strud
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Re: Swirl injector for 4" hybird

Postby strud » Fri Nov 22, 2019 4:06 pm

Photos of a quick low pressure water flow test

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These make it much easier to visualize what is going on.

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SpaceManMat
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Re: Swirl injector for 4" hybird

Postby SpaceManMat » Sat Nov 23, 2019 5:02 pm

Great result. Is it just instability or does this improve efficiency as well?
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

strud
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Re: Swirl injector for 4" hybird

Postby strud » Sat Nov 23, 2019 11:12 pm

Hi Matt,

Combustion efficiencies have been shown to go from around 85% to to 95%, however to gain this much improvement a post combustion chamber/zone is required.

Some example papers discussing this :

https://www.researchgate.net/publication/285630278_Swirl_Injection_Effects_on_Hybrid_Rocket_Motors

https://repository.asu.edu/attachments/110620/content/Summers_asu_0010N_12903.pdf

http://downloads.hindawi.com/journals/ijae/2017/3174140.pdf/

from the above paper:
"The test results show that the fuel grain regression rate has been improved by applying tangential oxidizer injection at the head of the motor. Besides, it was seen that combustion efficiency of motors with the swirl flow was about 10 percent more than motors with axial flow."

strud
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Re: Swirl injector for 4" hybird

Postby strud » Tue Dec 03, 2019 3:30 pm

Copper washers installed and injectors drilled to 1.6mm

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